With the explosive increase in demands of broad band multimedia communication services of the Internet, video delivery, etc., introduction of longer-distance, larger-capacity and higher-reliability wavelength division multiplexed optical fiber communication systems is progressing rapidly. In addition, also in subscriber systems, optical fiber access services are rapidly coming into widespread use. In such optical fiber-communication systems, the wavelength multiplexing technique of multiplexing signal lights of a plurality of different wavelengths to transmit is widely employed from a viewpoint of reduction of installation cost of optical fibers that are optical transmission lines and enhancement of the transmission band efficiency per line of optical fibers.
The optical modulator that features capability of high-speed optical modulation and a small signal-light wavelength dependency and whose unnecessary optical phase modulation (wavelength chirping) that causes deterioration of a received light waveform at the time of long distance signal transmission is also small is a key component of an optical transmitter intended for wavelength division optical fiber communication systems. An MZ type light intensity modulator that an optical wave guide type optical phase modulator is incorporated into the Mach-Zehnder (MZ) interferometer is suited for such a use. As an MZ light intensity modulator currently in practical use, one that has a structure in which the waveguide type optical phase modulator using a titanium (Ti) diffused planer optical waveguide as a base and an optical multi/demultiplexer are monolithically integrated on a lithium niobate (LiNbO3, LN) substrate, which is a typical electrooptic crystal, to constitute an MZ interferometer and electrodes for applying an electric field on this waveguide type optical phase modulator is provided in its vicinity is common. Although the LN-based an MZ optical modulator that is commercialized now has problems with respect to its dimensions (electrode length: about 5 cm, module length: about 15 cm) and a drive voltage (about 5 Vp-p), there is no practical optical modulator that surpasses this in terms of a high-speed long-distance optical transmission characteristic; therefore, it is widely used in optical transmitting units of various optical communication systems.
In a case where performing high-speed optical modulation using such an optical modulator, especially in a domain where a frequency of a modulation RF signal that is a driving signal exceeds 1 GHz, the propagation wavelength of the modulation RF signal becomes short to a degree that cannot be ignored to the electrode length of an optical phase modulator region in the LN optical modulator. For this reason, an electric potential distribution of an electrode structure that is means adapted to apply an electric field to the optical phase modulator cannot be assumed to be uniform in the longitudinal axis direction any more. In order to estimate optical modulation characteristics correctly in such a case, it is necessary to deal with this electrode structure and the modulation RF signal propagating there as a microwave transmission line and a traveling wave, respectively. In this case, at raveling wave electrode structure is required. In the traveling wave electrode structure, a contrive that brings the respective phase velocities vo and vm close to each other as much as possible (attaining velocity matching) so that an effective interaction length of a modulated optical signal propagating in the optical-phase modulator region and the modulation RF signal can be set as long as possible.
When realizing the optical phase modulator of an optical waveguide type and the MZ type optical modulator using III-V compound semiconductors, such as GaAs and InP that are useful in realizing light source elements, a technique where a single mode optical waveguide of a p-i-n type diode structure is constructed, and a reverse bias voltage is applied to this is used widely. The p-i-n type diode structure is constructed by forming an undoped core layer with a medium whose refractive index varies with the electric field intensity and sandwiching this with clad layers that have conductivities of p-type and n-type, respectively.
In the 1310 to 1650-nm band that is common in optical fiber communications, when providing an electrode in a single mode optical waveguide of a practical p-i-n type diode structure and dealing with this as a transmission line for the modulation RF signal, following problems arise. Due to an influence of the p-type semiconductor clad layer whose conductivity is generally low compared with that of the n-type, a (complex) characteristic impedance (absolute value thereof) of this transmission line that the modulation RF signal senses falls to the order of about 20Ω, about ½ time the representative characteristic impedance (50Ω) of the microwave circuit. As a result, there are problems in practical applications that include a restriction of a modulation frequency band due to reflection arising from impedance mismatch with a drive circuit etc. and bringing about of increase in driving current. Moreover, the effective complex refractive index nm (=c0/vm, c0: the velocity of light in free space) that the modulation RF signal senses also becomes around 7 in average for the same reason, and therefore, it will make a difference between itself and the effective refractive index no (=c0/vo, approximately 3.5, vo: the velocity of light in the medium) of the modulated optical signal, which is as much as about two times. Such velocity mismatch between modulated signal light and the modulation RF signal has a disadvantage that restricts the effective interaction length of the both and brings about the same problems related to the modulation frequency band and the drive voltage as that of the case where there is the impedance mismatch.
In this way, when adopting the traveling wave type electrode structure for an optical phase modulator or the electro-absorption type light intensity modulator, the p-i-n type diode structure involves a problem in attaining reduction in an operation voltage and a broad band.
On the other hand, the n-i-n type layered structure has a structure in which the p-type semiconductor clad layer that is the ground of the above-mentioned problems is replaced by an n-type semiconductor clad layer having the inverse conductivity. With the n-i-n type layered structure, it is possible to suppress the above-mentioned impedance mismatch and velocity mismatch to be small intrinsically and thereby, compatibility of the drive voltage amplitude reduction by elongating the element and attainment of broader band can be expected. These features are suitable to the traveling wave electrode structure that is advantageous when realizing a low-voltage and high-speed modulation operation of an optical modulator that is based on a semiconductor optical waveguide element, such as an electro-absorption optical modulator and an optical phase modulator. However, since if the bias voltage is applied to the both ends of the n-i-n layered structure as it is, electrons will be injected into the undoped layer and it becomes impossible to apply the electric field to the undoped layer, it is necessary to modify the structure into an n-SI-i-n type layered structure that sandwiches a semi insulating (SI) semiconductor (SI) layer practically. As an example applying this layered structure, an InP based semiconductor MZ light intensity modulator is reported (Kikuchi et al., “Low Voltage Drive 40 Gb/s Semiconductor Mach-Zehnder Modulator,” Institute of Electronics, Information and Communication Engineers, IEICE Tech. Rep., Optical Electronics, OPE2005-95, Nov. 3-4, 2005, hereinafter described as Non-patent Document 1).
The followings are documents pertaining to the optical modulator: Japanese Laid-Open Patent Application JP-P2001-235713A (Patent Document 1), Japanese Laid-Open Patent Application JP-P2002-139717A (Patent Document 2), and Japanese Laid-Open Patent Application JP-P1999-133366A (Patent Document 3).